The present invention relates to a magnetic resonance (MR) imaging apparatus and is particularly related to an apparatus that corrects image data samples to compensate for the motion of an object being imaged to provide a volumetric image larger than a restricted region of a main magnetic field. In magnetic resonance imaging, the subject to be imaged is positioned in a strong magnetic field, for example, produced in the bore of a superconducting electromagnet, and the protons of hydrogen atoms (and of other MR active nuclei) in water and fat align parallel and anti-parallel to the main magnetic field, precessing around the direction of the field at the Larmor frequency.
A transmit coil applies pulses of r.f. energy at the Larmor frequency in a direction orthogonal to the main field to excite precessing nuclei to resonance, which results in the net magnetisation of all MR active nuclei being flipped from the direction of the main magnetic field into a direction having a transverse component in which it can be detected by the use of a receive coil.
The received signal can be spatially encoded to produce two-dimensional (slice) or three-dimensional (slab) information about the distribution of MR active nuclei and hence of water and tissue.
The received signal can be confined to a slice of the patient in the following way.
Referring to FIGS. 1 and 2, a superconducting electromagnet 1 (seen in side view in FIG. 1 and in end view in FIG. 2) has a bore 2 for receiving a patient supported on a couch 3 which can be slid into the bore of the magnet from a position outside. By making the strength of the main magnetic field vary along the length of the bore (the z-axis) using magnetic field gradient coils to increase and/or decrease the main magnetic field, it is possible to excite MR active nuclei (the remainder of this description refers hydrogen nuclei as an example) confined to a slice orthogonal to the z-direction, since the Larmor frequency depends upon the strength of the magnetic field, and the frequency of the r.f. pulse can be chosen to correspond to that frequency. (Gradients could equally be set up to excite slices orthogonal to the x or y axes).
Spatial encoding of the slice can be produced by x and y magnetic field gradient coils which alter the strength, but not the direction, of the main magnetic field, in the x and y-directions. The frequency and phase information in the received signal can be analysed to map the distribution of the hydrogen nuclei in the plane of the slice.
Slab (or three-dimensional) imaging can be performed by commonly exciting a region with the r.f. excitation pulses that is to be divided into a contiguous series of slices using the phase of the received signal to distinguish between these different slices.
It is also known to image contiguous slabs, particularly when attempting to trace vessels such as blood vessels. In one known MRI apparatus, in order to be able to image along the length of the leg, three separate slab images are recorded. The patient is moved to three separate axial positions for the data to be captured. The three images are then joined together. However, there are discontinuities in the images when viewed in a projection normal to the slab boundaries, which is just the direction in which vessels would be viewed in.
It might be wondered why the three multi-slab images could not be acquired without moving the patient, since the length of the bore of the superconducting electromagnet is typically much greater than the length of a leg. However, the region of good field, over which the field is sufficiently uniform to obtain acceptably undistorted images is much smaller than the overall length of the bore. Typically, the region of good field would be an approximately spherical volume 4 in the centre of the bore.
While it is medically desirable to be able to collect three-dimensional image data from an elongated region, the trend in terms of electromagnets makes this more difficult to achieve, since patients find shorter magnets to be more acceptable, and shorter magnets have a region of good field 5 which is compressed in the axial direction, although not in directions at right angles.
The invention provides magnetic resonance imaging apparatus comprising means for applying r.f. excitation pulses to, and collecting volumetric data samples from, a restricted region of the main field of the apparatus, transforming the data samples to form a volumetric image of the restricted region, including means for advancing a patient support continuously through the restricted region, and means for correcting the data samples to compensate for the motion so that the volumetric image formed is of greater length than that of the restricted region.
The invention enables a shorter electromagnet with a restricted length of good field to produce extended three-dimensional images.
Advantageously, volumetric data samples are encoded with secondary phase encoding in the direction of motion of the patient support and with primary phase encoding which may have a greater number of increments in a transverse direction, the correcting means being arranged to compensate successive sets of samples of all secondary phases produced at each primary phase for the motion.
The present invention provides the foregoing and other features hereinafter described and particularly pointed out in the claims. The following description and accompanying drawings set forth certain illustrative embodiments of the invention. It is to be appreciated that different embodiments of the invention may take form in various components and arrangements of components. These described embodiments being indicative of but a few of the various ways in which the principles of the invention may be employed. The drawings are only for the purpose of illustrating a preferred embodiment and are not to be construed as limiting the invention.